Atmospheric Pressure

I'm about to put myself in the "WTF do you mean" category again!

Here I sit wondering about life and what the hell 15 #/in.^2 really means?

I'm almost affraid to read the replies in fear of looking like a complete blonde!

So, tell me, am I lifting ~ 15 pounds of 'air' per inch squared at sea level as I
raise my hands to the sky?

Is atmospheric pressure measured in a different type of POUND than a weight lifter's
barbell?

Maybe there is 15 pounds/in.^2 around me and I'm able to displace this air
with ease, but I'm not feeling the "weight" because there is equal pressure
around me?

:think:

you feel the weight as pressure on your skin. its pushing on you from every side, so you just dont have a point of refrence to compare it to.

while it is applying 14.7lbs of pressure per square inch on you. you wont feel "weight".. but you will feel resistance if you try to "push the air out of the way" with motion...


if you want a point of refrance, just stick some kind of vacume on yourself.... athough the pressure will cause the blood to rush there..... just tell everyone its a hicky. :P

http://school.discovery.com/lessonplans/programs/forcesandmotion/

;)

-Mindgame

I always understood that a lower barametric pressure or lower pressure, Will always help your car run faster. The lower the pressure, The easier it is for your car to move through the air. If there is less pressure then it would be easier for a car to move through the air, Almost having less resistance. At least, This is how I understand it.

I always understood that a lower barametric pressure or lower pressure, Will always help your car run faster. The lower the pressure, The easier it is for your car to move through the air. If there is less pressure then it would be easier for a car to move through the air, Almost having less resistance. At least, This is how I understand it.
You couldn't be more wrong. The amount of air that flows into the cylinder when the piston drops is directly proportional to the barometric pressure. Add 10% more pressure, and you have 10% more air in the cylinder, add 10% extra fuel, and you make 10% extra HP.

The effect of the air pressure (density) on the aerodynamic drag pales by comparison. Take your 14-second car to Denver, and see how quick it runs the 1/4-mile in that nice thin, low pressure air.... :D

I'm about to put myself in the "WTF do you mean" category again!

Here I sit wondering about life and what the hell 15 #/in.^2 really means?

I'm almost affraid to read the replies in fear of looking like a complete blonde!

So, tell me, am I lifting ~ 15 pounds of 'air' per inch squared at sea level as I
raise my hands to the sky?

Is atmospheric pressure measured in a different type of POUND than a weight lifter's
barbell?

Maybe there is 15 pounds/in.^2 around me and I'm able to displace this air
with ease, but I'm not feeling the "weight" because there is equal pressure
around me?

:think:

We live in a "sea of air" which goes from the surface to maybe 60 miles or so high. Because air is a fluid (gas) the upper molecules push on the lower ones so the total push or pressure at sea level (earth's surface) is the combined weight of all the molecules stacked about 6o miles deep. On a standard day, that's about 14.7 lbs of "push" or force per square inch. IOW, a 1 inch square column of air from sea level to the upper end of the atmosphere weights 14.7 lbs. This is barometric pressure.

You've felt the pressure change dramatically when you swim under water. Because water is denser than air, a few feet of water (I'm too lazy to do the conversion now) can vary the pressure 14.7 lbs.

Now about half of the air (and therefore pressure) is in the first 18,000 ft. above sea level. There's really not much air above 80-90000 ft. Some aircraft can fly at 80,000, but to get enough lift from the sparce number of air molecules they have to go fast so that lots of molecules pass by the wing.
The closer to sea level the more densely the molecules are packed together.

Yes, that 14.7 psi pushes in every direction. It's also inside you pushing out, or else you'd collapse.

An NA engine inhales air by creating a slightly lower pressure inside the cylinder during the intake stroke. The higher (relatively) atmospheric pressure forces the air into the lower pressure cylinder to balance the pressure. The engine doesn't really suck air in, it just lowers the pressure inside a little and the atmosphere does the rest. Your lungs do basically the same thing.

Global weather systems have high pressure and low pressure areas. Guess what: air flows from a high pressure area to a low pressure area. We call that airflow.......what? :) (2 point question)

FWIW, higher pressure means more molecules per a given volume, so higher barometric pressure means more air molecules injested and more power. We normally correct dyno data to some given pressure like 29.92 in. hg. which is about 14.7 psi. If barometric pressure in the test cell is higher than the standard pressure, corrected power is lower than measured and if test pressure is lower, corrected power is higher than observed.

The lower pressure air does decrease aero drag, but generally the lower power it provides makes a bigger difference and makes the vehicle slower, not faster. [oops! Injuneer beat me to it.]

MG is right; this should have been covered in earth science or some such subject where many of us were reading car magazines. :) (negative 2 points if that was you)

FWIW, those "little pages" of Rod & Custom of the 50's and 60's easily fit inside almost any text book. Even the nuns didn't find them.

My rambling $.02

You couldn't be more wrong. The amount of air that flows into the cylinder when the piston drops is directly proportional to the barometric pressure. Add 10% more pressure, and you have 10% more air in the cylinder, add 10% extra fuel, and you make 10% extra HP.

The effect of the air pressure (density) on the aerodynamic drag pales by comparison. Take your 14-second car to Denver, and see how quick it runs the 1/4-mile in that nice thin, low pressure air.... :D


I think Im thinking backwards, More barometric pressure means more air in cylinder right? So you want a higher pressure?
Alright, Another thought, Pressure relating to altitude and Map sensor readings?
Will a map sensor read closer to 100 kpa if the pressure is higher, In other words, The higher the altitude the lower the Barometric pressure? So if you were at 4000 ft. elevation like me, You would only pull 88-90 kpa due to the fact of thinner air and less pressure, Meaning less air and fuel in the cylinder. Explaning why cars run better at sea level, Higher pressure = more air and fuel in cylinder.

I can attempt to answer Kryckter's question.

Think of higher pressure meaning, more weight of air. If the barometer is high,
imagine 60 lbs of air sitting on the piston at top dead center (TDC), as opposed
to 30 lbs of air..

Once the piston moves downward and the valve opens, there is a low pressure
area created in the cylinder and the higher weight of air comes crashing into
the chamber.

This low pressure area depends on several things, including intake runner
volume, piston speed, displacement, RPM, engine resonance, etc.

The lower the pressure inside the cylinder, the greater chance of cramming
more air into the cylinder.

If the pressue inside the cylinder was higher than 14.7 PSI at the piston moved
downward at sea level, there would be little to nothing gained (which technically
should never happen unless the exhaust valve went on vacation).

As for my third thought in the first post, I now see the light thanks to many of
your replies!

(OldSS: My car magazines were substitued for Playboy at the time! :D )

I can attempt to answer Kryckter's question.

Think of higher pressure meaning, more weight of air. If the barometer is high,
imagine 60 lbs of air sitting on the piston at top dead center (TDC), as opposed
to 30 lbs of air..

Once the piston moves downward and the valve opens, there is a low pressure
area created in the cylinder and the higher weight of air comes crashing into
the chamber.

This low pressure area depends on several things, including intake runner
volume, piston speed, displacement, RPM, engine resonance, etc.

The lower the pressure inside the cylinder, the greater chance of cramming
more air into the cylinder.

If the pressue inside the cylinder was higher than 14.7 PSI at the piston moved
downward at sea level, there would be little to nothing gained (which technically
should never happen unless the exhaust valve went on vacation).

As for my third thought in the first post, I now see the light thanks to many of
your replies!

(OldSS: My car magazines were substitued for Playboy at the time! :D )

The motor will ingest a certain VOLUME of air over a given time. Air is a gas, so it compressible. When it is under pressure, there is a greater MASS (greater number of molecules) in the same volume. So, higher barometric pressure = more air molecules per volume of air = more oxygen molecules to combine with more fuel molecules = more power (if the fuel needed is provided).

Ever think about why it's a MASS air sensor and not a VOLUME air sensor? What counts is the mass of air the motor takes in, not the volume.

Rich

So, In theory, People will have more Barometric Air pressure at sea level vs. me at 4000 ft. elevation? Cause from what I am understanding , The MAP sensor will read closer to 100 kpa at sea level if everything is right vs. me running 88-90 kpa at my elevation. Is most of this due to thinner air and the lower Barometric pressure reading? In other terms, This is why cars run better at sea level. More air molecules in the air, Denser air. And This would be why most cars at higher elevation will still pull vacuum in the intake when WOT due to thinner air and less air molecules. In turn...Less HP!

More air the better.

You couldn't be more wrong. The amount of air that flows into the cylinder when the piston drops is directly proportional to the barometric pressure. Add 10% more pressure, and you have 10% more air in the cylinder, add 10% extra fuel, and you make 10% extra HP.

The effect of the air pressure (density) on the aerodynamic drag pales by comparison. Take your 14-second car to Denver, and see how quick it runs the 1/4-mile in that nice thin, low pressure air.... :D
What if we were driving/racing an electric powered car? It seems to me that an electric car might actually run quicker up in Denver than it would down here in Houston due to less aero drag. :think:

What if we were driving/racing an electric powered car? It seems to me that an electric car might actually run quicker up in Denver than it would down here in Houston. :think:

That was my original view on the subject. But The HP amount of the denser air is far more substantial than the denser air slower you down.
Take in example for the electric car idea being faster in Denver, Look at baseball, it is alot easier to hit a home run in Denver than it is Houston, Because of thinner air. But when you compare the thinner air to the amount of HP you get from denser air, It is not comparable. More air molecules in the engine will help you out alot more than the thinner air letting the car move easier.

Take in example for the electric car idea being faster in Denver, Look at baseball, it is alot easier to hit a home run in Denver than it is Houston, Because of thinner air. But when you compare the thinner air to the amount of HP you get from denser air, It is not comparable. More air molecules in the engine will help you out alot more than the thinner air letting the car move easier.


Yeah, but trying to stretch a single into a double about winds any lowlander player! A few miles south of Denver at the top of Pikes Peak (14000 or so ft), just walking acros the parking lot is a challenge!

Ahh the joys of racing at altitude. The engine makes less power and you don't break as many parts.

Yes you want high barometric pressure but you also want low humidity and cool temperatures. These 3 plus the altitude create a density altitude number. Someone says they raced a couple of weeks apart and the temperature and humidity were the same but the ran slower. Chances are the barometric pressure was much lower causing the slower runs.

There are pros and cons to high pressure. Although there are more air molecules to burn, the air is also denser meaning it's thicker to push through. You make more HP from the dense air but you eat up some of that extra HP just trying to push through it.

Humidity is another factor. Water molecules will take up some of the space the air is trying to use. There is a lot more water in the air at 50% humidity and 80* than there is at 50% humidity and 60* because warm air can hold more water. You need to know what the dew point is to really know how much water is in the air. Basically if the humidity is high and the temperature is high, the engine is trying to burn a lot of water vapor along with the air.

Using a power adder sort of eliminates the need for high barometric pressure since the power adder is forcing more air into the engine, it doesn't rely on the engine's ability to suck available air in to mix with the fuel.

Remember superchargers and NOS were used in WWII so that the fighter planes wouldn't lose power at high altitude for the same reasons.

Zero-to-69,

Please don't take this the wrong way. I like your enthusiasm for learning... that's a good thing! So I will recommend that you go down to your local bookstore, or better yet a discount book store, and pick up a 1st year textbook on Physics. They are dirt cheap... people finish their course and give the book away because they "can't stand Physics". You know what they say about one man's junk..... :)

I think you'd get alot more out of that then anyone could possibly post here on the board. Hey.. it all relates to engines and going fast!

-Mindgame

Zero-to-69,

Please don't take this the wrong way. I like your enthusiasm for learning... that's a good thing! So I will recommend that you go down to your local bookstore, or better yet a discount book store, and pick up a 1st year textbook on Physics. They are dirt cheap... people finish their course and give the book away because they "can't stand Physics". You know what they say about one man's junk..... :)

I think you'd get alot more out of that then anyone could possibly post here on the board. Hey.. it all relates to engines and going fast!

-Mindgame
Excellent suggestion!

I recently found a good college Physics text at a book sale held by the local public library. $2 for a $million worth of info.

I never take offense! I play here with the big boys to learn, not to be turned
away.

It wont be the first time I purchased a book from a suggestion. Scientific Design of Exhaust and Intake Systems proved to be an excellent source.

The strange thing is, my highschool Physics didn't go into depth, or even
touch on a few of the points I've posted in this forum.

I remember lessons on Mechanical advantage (pulley's, gears, levers), sound pressure, Bernoulli's principle, basic Ohm's law. In College, all of the Physics
related to Electric/Electronics.

I'll have a look through my old text out of curiousity; I doubt it will mention
anything we're discussing here.

Your MAP sensor performs two functions. First it tells the computer what the barometric pressure is. It can range anywhere from 81kPa at 6,000-ft altitude, to 101kPa at sea level. That's "standard" barometer.... depending on the weather, you will see numbers a bit above or below those.

When you start the engine, the MAP sensor is now reading the pressure inside the intake manifold, which is equal to the barometric pressure minus any pressure "losses" in the intake track. At idle, you are intentionally putting an obstruction (closed throttle blades) in the inlet air path to reduce air flow, decreasing the pressure available. When you fully open the throttle blades, MAP should approach barometric pressure, with the differences representing the inefficiencies (friction losses) of your inlet air ducting/filter/etc.

For a comparison of "standard" barometric readings vs. altitude:

ELEV - - - - -STD BAROMETER
Feet - - - -"Hg - - -PSIa - - kPa
0 - - - - -29.92 - - 14.7 - - -101.3
1,000 - - 28.86 - - 14.2 - - - 97.7
2,000 - - 27.82 - - 13.7 - - - 94.2
3,000 - - 26.81 - - 13.2 - - - 90.8
4,000 - - 25.84 - - 12.7 - - - 87.5
5,000 - - 24.89 - - 12.2 - - - 84.3
6,000 - - 23.98 - - 11.8 - - - 81.2

SNIP
For a comparison of "standard" barometric readings vs. altitude:

ELEV - - - - -STD BAROMETER
Feet - - - -"Hg - - -PSIa - - kPa
0 - - - - -29.92 - - 14.7 - - -101.3
1,000 - - 28.86 - - 14.2 - - - 97.7
2,000 - - 27.82 - - 13.7 - - - 94.2
3,000 - - 26.81 - - 13.2 - - - 90.8
4,000 - - 25.84 - - 12.7 - - - 87.5
5,000 - - 24.89 - - 12.2 - - - 84.3
6,000 - - 23.98 - - 11.8 - - - 81.2

This table is quite revealing. Want to get a feel for how much power is lost going from sea level to 6,000ft? Think about about how much power a blower car would lose going from 14.7 to 11.8psi of boost, almost 3psi differerence. Look at the PSI column and you can see that same difference in air density from 0 - 6,000ft.

OTOH, it's very interesting that human performance, in acclimated very fit individuals like bike racers, can improve at high altitude. Human performance is also intimately related to "air flow" - how much oxygen can be delivered to the muscles by the cardiovascular/respiratory systems. And anyone sho has gone suddenly from the flatlands to high altitiude knows how debilitating that is due to the sudden decreased oxygen content of the air and lower barometric pressure. But OTOH, most bicycle speed records have been set at relatively high altitude, illustrating the iomportance of aerodynamic drag. Of course, the riders are extremely fit and also need to acclimate before trying for a record.

In the realm of internal combustion I know that piston driven aircraft have an optimum altitiude (air density) for maximum speed. I assume that is the altitiude where the loss of hp from lower oxygen content is balanced by the lower drag from the thinner air. I am not enough of an engineer to back this up, but I suspect that wrt automobiles, a similar phenomenon exists wrt top speed, where aerodynamic drag is much more important than in the 1/4m. For a given combo there would be an optimum air density for top speed. But in the 1/4m, hp is king and the denser the air the better.

Rich

Tino,
Everything from your intial post in this thread was covered in your first physics textbook. Gravity, pressure, density, altitude.... and the beat goes on.

It seems you are really searching for concepts in your responses (other threads too) and the only problem with that, as I see it anyways, is the lack of a good foundational understanding. I don't think I'm the only one who sees this. :)

The desire to learn can't be bought and you've got that in spades. Just need to channel your efforts in the right direction. Now, I'm gonna go grab my physics book and try to brush up on a few things I've forgotten. :lol:

-Mindgame

Excellent suggestion!

I recently found a good college Physics text at a book sale held by the local public library. $2 for a $million worth of info.

They basically give used text books away at the discount book stores. Especially when new books are adopted for a new curriculum. I find books all the time on thermo, materials science, statics, etc.. dirt cheap. Can't beat that.

-Mindgame

Very good points, Rich.

I suspect that at TF or FC speeds, the few tons of downforce results in so much induced drag (the drag that results from creating lift or downforce) that becomes the speed limiting factor. In order to produce the necessary downforce at altitude (Denver), lots of angle of attack is required on the wing and induced drag is even higher.

For land speed record cars, you pretty much have to accept the altitude of the few places there are to run.

Air density and the power vs. drag equation applies to jet aircraft dramatically. Many jets can exceed Mach 1 or Mach 2 at altitudes at 35-40,000 ft. where the air pressure is about 3 psi. Sure power is way down, but so is drag. The Concorde did M2 @ about 65-70000 ft, where there atmospheric pressure is about 1 psi! The SR-71 family of Blackbirds did M3.2 or so at 80,000+ where the pressure is about .3-.4 psi. Here the drag is minimal, but the few air molecules there are strike the aircraft at over 3000 ft per second and the friction that causes heats everything up to 250-650F.

Very few jets can fly at M1+ at sea level, where the aero drag means a lot. Power required to ovecome aero drag is proportional to the cube (^3) of the speed. As the air density drops at altitude, the drag drops at the same rate.

AFAIK, piston engine aircraft speed records have been set at near sea level. In the 500-600 mph range, power is probably more important than drag reduction from lower atmospheric pressure.

My $.02

Your MAP sensor performs two functions. First it tells the computer what the barometric pressure is. It can range anywhere from 81kPa at 6,000-ft altitude, to 101kPa at sea level. That's "standard" barometer.... depending on the weather, you will see numbers a bit above or below those.

When you start the engine, the MAP sensor is now reading the pressure inside the intake manifold, which is equal to the barometric pressure minus any pressure "losses" in the intake track. At idle, you are intentionally putting an obstruction (closed throttle blades) in the inlet air path to reduce air flow, decreasing the pressure available. When you fully open the throttle blades, MAP should approach barometric pressure, with the differences representing the inefficiencies (friction losses) of your inlet air ducting/filter/etc.

For a comparison of "standard" barometric readings vs. altitude:

ELEV - - - - -STD BAROMETER
Feet - - - -"Hg - - -PSIa - - kPa
0 - - - - -29.92 - - 14.7 - - -101.3
1,000 - - 28.86 - - 14.2 - - - 97.7
2,000 - - 27.82 - - 13.7 - - - 94.2
3,000 - - 26.81 - - 13.2 - - - 90.8
4,000 - - 25.84 - - 12.7 - - - 87.5
5,000 - - 24.89 - - 12.2 - - - 84.3
6,000 - - 23.98 - - 11.8 - - - 81.2

Thanks for the info! That pretty much the idea I had but the table is some very interesting facts. It is amazing how much the altitude and air pressure can affect our cars performance. It would be nice to see some dyno numbers of different KPA readings and not have the numbers corrected to see how much this stuff is actually affecting some of the people like me that see a regular of 88 KPA.

Hi again,

Sorry if this came across as a classroom Physic's lesson, but I wasn't sure
how to word the question as it related to engines.

Grade 12, general Physics in Ontario doesn't seem to cut it (year 1990).
I regret not taking the advanced level course, but at the time it was not required
to enter Trade school, or College.

I've come a long way since buying my Camaro. I couldn't change a tire
back at the age of 15, now I'm making horsepower from an oil pump...who
would have thought?

One day, I'll post a reply to a thread a surprise all of you. Once I begin
connecting all the pieces, the picture is going to be beautiful.

Thanks for the suggestions. If you have any intermediate level reading
for Physics, Fluid Dynamics, Thermodynamics, please send them my way!